The present invention generally relates to a method and an apparatus for controlling the speed of a moving body, and particularly relates to a method and an apparatus for controlling the speed of a moving body suitable for use for controlled equipment which cannot be allowed to fluctuate in its speed in operation, such as, for example, a motor of a VTR (video tape recorder).
A VTR driving motor is desired to have a speed which can be kept constant. If a fluctuation in speed (such as irregular rotation, speed ripple, torque ripple) occurs in such a VTR driving motor, a picture is disturbed, so that reliability and quality required for the VTR deteriorate extremely. Although mainly DC motors have been used in such equipment conventionally, brushless motors the speed of which can be changed desirely and simply are often employed. A brushless motor has no mechanical brush, so that various problems due to abrasion or abrasion powder of a brush and/or a commutator do not occurs in the motor. In a brushless motor of the 120-degree current-conduction system, however, the number of flux interlinkage of a current-conducting coil varies depending on the position of a rotor to thereby cause a torque ripple and hence irregular rotation (speed fluctuation) in operation. Since this irregular rotation varies depending on the gain of a speed control system, it is possible to reduce the irregular rotation by making the gain of the speed control system larger.
If the gain of the speed control system is made too large, however, the speed control system becomes unstable enough to rather increase the irregular rotation. Although an optimum value of gain for reducing the irregular rotation exists in fact, the optimum value of gain in such a speed control system giving an influence on the irregular rotation is affected by a torque constant and/or a resistance of a motor, and further by a gain of an analog system if such an analog system is included in the speed control system. On the other hand, since the optimum value of gain varies depending on variations in temperature, in resistance values of used resistors, and so on, the optimum gain takes different values in individual control systems.
Under the above-mentioned circumstances, a conventional speed control system has been arranged such that the value of a control gain is selected so as not to make the speed control system unstable even under the worst conditions. In the case where a speed control apparatus is turned on under the condition of normal temperature or in the case where a speed control apparatus has no variations in resistance, however, there rather arises a disadvantage that irregular rotation may occur more or less.
JP-A-55-79694 discloses a motor control circuit wherein the control gain is changed in accordance with the position of the rotar, to reduce torque ripple.